It is known that incomplete combustion of certain heavy hydrocarbon compounds, such as heavy oils, diesel fuel and the like may lead to particulate formation (e.g., soot). In the operation of internal combustion engines, “smoking” of the engine leads to ambient air pollution although the carbon monoxide and hydrocarbon emission of the exhaust gases itself are relatively low. It is further known to provide a particulate sensor system for detecting the level of particulate concentration emitted from an exhaust gas, as seen by reference to U.S. Pat. No. 6,634,210 entitled PARTICULATE SENSOR SYSTEM issued to Bosch et al, assigned to the common assignee of the present invention and hereby incorporated by reference in its entirety.
Bosch et al. disclose a particulate sensor system having a pair of spaced apart sensing electrodes disposed on a substrate. The sensing electrodes are coupled to a measurement circuit by way of electrically conductive leads. The operating principle of the particulate sensor is based on the conductivity of the particulates (e.g., soot) deposited on (or over) the sensing electrodes. The electrical resistance between the sensing electrodes is relatively high when the sensor is clean but such resistance decreases as soot particulates accumulate. Bosch et al. further disclose a heater that is selectively activated to burn off the soot particulates to “reset” the sensor to a known, base “clean” state. However, for diagnostic purposes, there is some difficulty distinguishing two states: (i) a faulty state such as when there is an electrical open circuit in the wiring leads, which presents as a very high resistance between the sensing electrodes, and (ii) a normal state, such as when a sensor has just been cleaned, which also presents as a very high resistance. In the former case, there is a wiring fault while in the latter case no faults exist.
In some applications, soot accumulation is very slow or only occurs when there is a malfunction in an exhaust after-treatment device, such as a diesel particulate filter. However, the integrity of the wiring leads to the sensor must still be assured even in the absence of soot.
There is therefore a need for a particulate sensor system that minimizes or eliminates one or more of the problems noted above.